Method for producing a plug for hot tube-making

ABSTRACT

A plug for hot tube-making having a sprayed coating resistant to peeling is provided. A method for producing a plug for hot tube-making according to the present embodiment includes a step of preparing a plug whose surface includes a sprayed coating formed thereon, the sprayed coating containing iron and an iron oxide; and a heat treatment step during which the plug is kept at 400° C. to 550° C. for 5 to 60 minutes.

TECHNICAL FIELD

The present invention relates to a method for producing a plug for hottube-making (hereafter simply referred to as a plug), and moreparticularly to a method for producing a plug included in a piercer andan elongator.

BACKGROUND ART

The Mannesmann tube-making process is widely adopted as the method forproducing seamless tubes. In the Mannesmann tube making process, a roundbillet heated to around 1200° C. is piercing-rolled by a piercer. Thepiercer includes a pair of inclined rolls and a plug. The plug isdisposed between the inclined rolls of the pair and on a pass line. Thepiercer presses the round billet onto the plug while rotating the roundbillet in the circumferential direction by means of the inclined rolls,to piercing-roll the round billet into a hollow shell. Further, asneeded, the elongator elongates the hollow shell to expand the diameterand reduce the thickness thereof. The elongator has a similarconfiguration to that of the piercer, that is, a pair of inclined rollsand a plug.

As so far described, since the plug is used for piercing, and elongatinga round billet of high temperature to expand the diameter thereof, theplug is subject to high temperature and high interfacial pressure fromthe round billet. As a result, the surface of the plug is subject towear and scoring. Such wear and scoring will reduce the life of theplug.

To improve the life of the plug, a technique to form a sprayed coatingon the surface of the plug is proposed. For example, in WO2009/057471(Patent Document 1), arc spraying is performed with iron wire rod ontothe surface of the plug to form a sprayed coating. Patent Document 1describes that the sprayed coating improves the scoring resistance ofthe plug thereby increasing the life thereof.

DISCLOSURE OF THE INVENTION

However, the sprayed coating may have a poor adhesiveness to the plugbody. Poor adhesiveness will cause the sprayed coating to peel off. Ifthe sprayed coating peels off, inner surface flaws of the hollow shellbecome more likely to occur. Moreover, the life of the plug will alsodecline.

It is an object of the present invention to provide a method forproducing a plug having a sprayed coating which is resistant to peeling.

A method for producing a plug for hot tube-making according to thepresent embodiment includes a step of preparing a plug whose surfaceincludes a sprayed coating formed thereon, the sprayed coatingcontaining iron and an iron oxide; and a heat treatment step duringwhich the plug is kept at 400° C. to 550° C. for 5 to 60 minutes.

The method for producing a plug according to the present embodiment canproduce a plug whose sprayed coating is less likely to peel off.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a plug according to thepresent embodiment;

FIG. 2 is a diagram showing the relationship between the heat treatmenttemperature and the proportions of iron and iron oxide in the coating;

FIG. 3 is a cross sectional view of a specimen to be used for a test tomeasure tensile residual stress in the sprayed coating of the plug; and

FIG. 4 is a schematic diagram to explain the method of measuring tensileresidual stress by use of the specimen shown in FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereafter, referring to the drawings, an embodiment of the presentinvention will be described in detail. The like or corresponding partsin the drawings are given the like reference characters, withoutrepeating the description thereof.

The method for producing a plug according to the present embodimentincludes a step of preparing a plug whose surface includes a sprayedcoating formed thereon, the sprayed film containing iron and an ironoxide; and a heat treatment step during which the plug is kept at 400°C. to 550° C. for 5 to 60 minutes.

The iron oxide in the prepared sprayed coating of the plug is composeddominantly of wustite (FeO). Wustite has a relatively low adhesiveness.Retaining the plug, on which such sprayed coating is formed, at 400° C.to 550° C. for 5 to 60 minutes causes the wustite in the sprayed coatingto transform into magnetite (Fe₂O₃). Magnetite has a higher adhesivenessthan that of wustite. Therefore, the adhesiveness of the sprayed coatingto the plug body is improved, and the sprayed coating becomes lesslikely to peel off.

Further, tensile residual stress remains in the sprayed coating, whichhas been formed on the surface of the plug body by spraying. If a plugin which tensile residual stress still remains in the sprayed coating isused for piercing-rolling, the sprayed coating becomes more likely topeel off due to the tensile residual stress. In the present embodiment,the plug including a sprayed coating formed thereon is subjected to aheat treatment at the above described conditions (at 400° C. to 550° C.for 5 to 60 minutes). As a result, the tensile residual stress in thesprayed coating is reduced, and the sprayed coating becomes less likelyto peel off.

Hereafter, the details of the method for producing a plug according tothe present embodiment will be described.

[Method for Producing a Plug]

The method for producing a plug according to the present embodimentproduces a plug for hot tube-making. The plug for hot tube-making can beutilized for a piercer and an elongator. The method for producing a plugof the present embodiment includes a step (preparation step) ofpreparing a plug having a sprayed coating, and a step (heat treatmentstep) of heat treating the plug.

[Preparation Step]

In the preparation step, a plug 10 as shown in FIG. 1 is prepared.

The plug 10 includes a plug body 11 and a sprayed coating 12. The plugbody 11 has, for example, a well-known shape and material.

The sprayed coating 12 contains iron (Fe) and an iron oxide. Preferably,the sprayed coating 12 consists of iron, an iron oxide, and impurities.The sprayed coating 12 consisting of iron, an iron oxide, and impuritiesis formed by, for example, arc spraying as follows. An iron wire rodwhich provides a spraying material, and an arc spraying apparatus areprepared. The arc spraying apparatus includes a spraying nozzle. Thespraying nozzle blows out the spraying material which is melted by arc,with compressed air or nitrogen gas.

The iron wire rod is arc sprayed by the arc spraying apparatus to form asprayed coating 12 on the surface of the plug body 11. By adjusting thedistance (hereafter referred to as a spraying distance) between thespraying nozzle of the arc spraying apparatus and the plug body 11, itis possible to adjust the content of the iron oxide in the sprayedcoating. The spraying distance is, for example, 200 to 1000 mm. The arcspraying apparatus can adjust the position of the spraying nozzle. Notethat the sprayed coating 12 may be formed by other spraying processesthan the above described arc spraying.

Since the sprayed coating 12 contains iron, which provides the matrix,and an iron oxide, it has wear resistance and thermal insulationperformance. Thereby, the wear and melting loss of the plug body 11 aresuppressed.

However, the iron oxide in the sprayed coating 12 formed by spraying ismostly wustite (FeO). Wustite has poor adhesiveness to the plug body 11.Therefore, the sprayed coating 12 may peel off from the plug body 11.Further, since the sprayed coating 12 is formed by spraying, tensileresidual stress remains in the sprayed coating 12 after it is cooled.The tensile residual stress reduces the adhesiveness of the sprayedcoating 12. Thereby, the tensile residual stress makes the sprayedcoating 12 more likely to peel off from the plug body 11.

Accordingly, to further increase the adhesiveness of the sprayed coatingto the plug body 12, the following heat treatment is conducted.

[Heat Treatment Step]

Heat treatment is conducted on the prepared plug 10, that is, the plug10 after the sprayed coating 12 is formed by spraying. To be specific,the plug 10 is put into a heat treatment furnace. The internaltemperature of the heat treatment furnace is kept at 400° C. to 550° C.As a result, the temperature of the plug 10 put into the heat treatmentfurnace will become 400° C. to 550° C. In the heat treatment furnace,the temperature of the plug 10 is retained at 400° C. to 550° C. for 5to 60 minutes. The plug 10 is taken out from the heat treatment furnaceafter the retention time has passed.

In the sprayed coating 12 of the plug 10, which is produced by the abovedescribed step, the transformation from wustite to magnetite takesplace, and the ratio of magnetite in the sprayed coating 12 increases.Further, the tensile residual stress decreases. Thereby, theadhesiveness of the sprayed coating 12 to the plug body 11 increases,and the sprayed coating 12 becomes less likely to peel off. Hereafter,such operation will be described in detail.

[Transformation from Wustite to Magnetite]

The iron oxide includes wustite (FeO), magnetite (Fe₃O₄), and hematite(Fe₂O₃). Among these, since magnetite is more consistent with iron thanwustite and hematite are, magnetite has a highest adhesiveness with theplug body 11. Further, the heat insulation performance of magnetite isequal to those of wustite and hematite. Therefore, to increase theadhesiveness of the sprayed coating of the plug 10, it is preferable toincrease the ratio of magnetite in the iron oxide of the sprayedcoating.

Employing a heat treatment temperature of 400° C. to 550° C. and a heattreatment time of 5 to 60 minutes will make it possible to reduce theratio of wustite and increase the ratio of magnetite in the sprayedcoating.

FIG. 2 is a diagram showing the relationship between the heat treatmenttemperature and the proportions of iron, which is the matrix, and eachiron oxide (wustite, magnetite, and hematite) in the coating. FIG. 2 wasresulted from the following method.

A plurality of thick plate test specimens (each of which has dimensionsof 20 mm×50 mm×10 mm) were prepared. For each test specimen, arcspraying was performed at the same condition by using an iron wire rodhaving the same composition, to form a sprayed coating of the samethickness (500 μm) in each test specimen.

After the sprayed coating was formed, the composition of the sprayedcoating of each test specimen was analyzed. To be specific, by an X-raydiffraction method (XRD method), peak intensities of iron and each oxide(wustite, magnetite, and hematite) in the sprayed coating of each testspecimen were determined. In the present discussion, for the sake ofconvenience, the heights of maximum peaks out of the obtained peakintensities were used as the indices for the ratios of iron and eachiron oxide in the sprayed coating. The ratios of iron and each ironoxide in the sprayed coating of each test specimen as sprayed were thesame for every test specimen.

Next, each test specimen was subjected to heat treatment. The heattreatment temperature was 400° C. to 650° C., and the heat treatmenttime was 60 minutes for every test specimen. The ratios of iron and ironoxides in the sprayed coating were analyzed by an XRD method on thesprayed coating of the test specimens after heat treatment to obtainFIG. 2. The ordinate in FIG. 2 indicates peak intensity ratios of ironand each iron oxide. The peak intensity ratio was defined by thefollowing formula.

Peak intensity ratio=Each maximum peak height of iron and ironoxide/total of maximum peak heights of iron and each iron oxide

A higher peak height ratio means a larger ratio of the content in thesprayed coating. With reference to FIG. 2, when the heat treatmenttemperature is not less than 400° C., the peak intensity ratio ofmagnetite remarkably increased and also the peak intensity ratio of iron(Fe) increased as the temperature rose.

Here, the chemical reaction formula of the transformation from wustiteto magnetite is as follows.

4FeO→Fe+Fe₃O₄

Referring to the above described chemical reaction formula and FIG. 2,when the heat treatment temperature is not less than 400° C., thetransformation from wustite to magnetite is facilitated, and the ironratio and magnetite ratio in the spray coating increase. On the otherhand, the wustite ratio decreases.

Further, when the heat treatment temperature is higher than 550° C., theiron ratio rapidly decreases and the hematite ratio increases as theheat treatment temperature rises. The decrease of the iron ratio isprobably due to that the oxidation of the sprayed coating itself hasprogressed causing the iron in the sprayed coating to transform intohematite.

As so far described, if the heat treatment temperature is 400° C. to550° C., the magnetite ratio and the iron ratio, which increase theadhesiveness with the plug body 11, will increase in the sprayed coating12. As a result, the adhesiveness of the sprayed coating 12 to the plugbody 11 will increase.

[Reduction of Tensile Residual Stress]

Conducting heat treatment at 400° C. to 550° C. will further reducetensile residual stress in the sprayed coating. Table 1 shows the levelof tensile residual stress (a deflection) when heat treatment isconducted at each condition (heat treatment temperature and heattreatment time). Table 1 was resulted from the following method.

TABLE 1 Heat treatment temperature Heat treatment time (minutes) (° C.)5 20 40 60 70 400 0.3 0.25 0.2 0.1 0.1 450 0.35 0.3 0.22 0.12 0.12 5000.3 0.3 0.2 0.16 0.16 550 0.3 0.25 0.22 0.2 0.15 600 0.25 0.25 0.2 0.150.15 650 0.3 0.2 0.15 0.1 0.1 Non-heat 0.55 treatment

A plurality of specimens 20 shown in FIG. 3 were prepared. Each specimen20 included a base 21, a base metal 22, a sprayed coating 23, and aplurality of bolts 24. The base 21 had dimensions of 20 mm width, 50 mmlength, and 10 mm thickness. The base metal 22 was disposed on the upperface of the base 22. The base metal 22 had a shape of 20 mm width, 50 mmlength, and 1 mm thickness. As shown in FIG. 3, each end portion(opposite side portions) of the base metal 22 was secured to the base 21with a bolt 24. A sprayed coating 23 was formed on the upper face of thebase metal 22. The sprayed coating 23 was formed by arc spraying, thecondition of which was the same as in FIG. 1, and the thickness thereofwas 500 μm.

Heat treatment was not conducted for one specimen 20, and heat treatmentwas conducted at various heat treatment temperatures (400 to 650° C.)and heat treatment times (5 to 70 minutes) shown in Table 1 for theother plurality of specimens 20. One of the bolts 24, which were securedto both end portions of the specimen 20 which was not subjected to heattreatment and the specimen 20 subjected to heat treatment, was detachedas shown in FIG. 4. At this moment, as shown in FIG. 4, the base metal22 and the sprayed coating 23 were deflected due to tensile residualstress. Then, as shown in FIG. 4, the distance from the upper face ofthe base 21 to the edge of the lower face of the base metal 22 wasdefined as a deflection FL (mm). It was judged that the deflection FLwas an index of tensile residual stress, and the larger the deflection,the larger the tensile residual stress.

Referring to Table 1, in the specimens 20 which were subjected to heattreatment, the deflection FL was smaller than in the specimen 20 whichwas not subjected to heat treatment, and thus tensile residual stresswas reduced. Further, in the specimens 20 subjected to heat treatment,the tensile residual stress decreased as the heat treatment timeincreases.

As so far described, retaining the sprayed coating 12 at a heattreatment temperature of 400° C. to 550° C. for 5 to 60 minutes willresult in an increase in the magnetite ratio in the sprayed coating 12,thereby increasing the adhesiveness of the sprayed coating 12 to theplug body 11. Further, the tensile residual stress of the sprayedcoating 12 also decreases. Therefore, in the plugs 10 subjected to heattreatment at the above described condition, the sprayed coating 12becomes less likely to peel off from the plug body 11.

When the heat treatment time is too short, the transformation fromwustite to magnetite will not proceed so that the adhesiveness of thesprayed coating 12 is not likely to increase. On the other hand, whenthe heat treatment time is too long, further oxidation will proceed inthe sprayed coating 12 so that the ratio of iron which is the matrix,declines and the ratio of iron oxides such as hematite increases.Therefore, the adhesiveness of the sprayed coating 12 to the plug body11 decreases on the contrary. Accordingly, the heat treatment time is 5to 60 minutes. A preferable lower limit of the heat treatment time willbe 10 minutes, and more preferably 20 minutes.

The heat treatment temperature is preferably 500° C.±25° C. (475° C. to525° C.).

In the plugs 10 which are produced in the above described steps (thepreparation step and the heat treatment step), the adhesiveness of thesprayed coating 12 to the plug body 11 increases. Therefore, the sprayedcoating 12 is less likely to peel off.

In the plugs 10 prepared in the above described preparation step, thesprayed coating 12 contains iron and iron oxides. However, the sprayedcoating 12 may further contain other oxides. For example, the sprayedcoating 12 may contain W oxide (WO₃) along with iron oxides. Forexample, by arc spraying an iron wire rod containing W, the sprayedcoating 12 will contain iron, iron oxides, and W oxide. Even with a plugon which such a sprayed coating is formed, the result of conducting theabove described heat treatment step will be that the magnetite ratio inthe sprayed coating increases, and the tensile residual stressdecreases. Therefore, the sprayed coating is less likely to peel off.

Examples

Each plug having a sprayed coating was heat treated at conditions oftest numbers 1 to 20 shown in Table 2. An anti-peeling property of thesprayed coating of the each plug produced was evaluated.

TABLE 2 Heat Heat treatment treatment Peeling Test temperature time ratenumber (° C.) (minutes) (%) 1 350 20 40 2 350 40 40 3 400 10 30 4 400 2020 5 400 40 20 6 400 60 20 7 400 80 50 8 500 10 30 9 500 20 10 10 500 4015 11 500 60 10 12 500 80 50 13 550 10 30 14 550 20 20 15 550 40 20 16550 60 20 17 550 80 40 18 600 20 50 19 600 40 50 20 — — 40

[Test Method]

Every plug of each test number had the shape shown in FIG. 1. A sprayedcoating of a thickness of 600 μm was formed on the surface of each plug.The sprayed coating was formed by arc spraying an iron wire rod. Theconditions of arc spraying of each plug were the same for all of them.Ten plugs were prepared for each test number.

Heat treatment was conducted on 10 plugs for each test number at theconditions (heat treatment temperature and heat treatment time) shown inTable 2. Note that the plug of test number 20 was not subjected to theheat treatment.

After the heat treatment, the plugs of test numbers 1 to 20 were used topiercing-roll round billets. The round billets had a chemicalcomposition corresponding to SUS304 of the JIS Standard, and each hadthe same diameter and length. Each plug of each test number was used topiercing-roll three round billets. At this moment, the heatingconditions and the piercing-roll conditions of each round billet werethe same for every test number.

After piercing-rolling three round billets, the surface of each plug wasvisually observed to judge the presence or absence of the peeling of thesprayed coating. Then, a peeling rate (%) was determined for each testnumber based on the following formula.

Peeling rate=number of plugs in which peeling is confirmed/10×100

In short; out of the 10 plugs of each test number, the rate of the plugsin which peeling is confirmed was defined as a peeling rate (%).

[Test Results]

Referring to Table 2, in test numbers 3 to 6, 8 to 11, and 13 to 16, theheat treatment temperature was within a range of 400° C. to 550° C. andthe heat treatment time was within a range of 5 to 60 minutes for eachtest number. As a result, the peeling rate was as low as not more than30%.

On the other hand, in test numbers 1 and 2, the heat treatmenttemperature was as low as 350° C. Thereby, the peeling rate was high.

In test numbers 7, 12, and 17, although the heat treatment temperaturewas appropriate, the heat treatment time was as long as 80 minutes.Thereby, the peeling rate was high.

In test numbers 18 and 19, the heat treatment temperature was as high as600° C. Thereby, the peeling rate was high.

In test number 20, no heat treatment was conducted. Thereby, the peelingrate was high.

Although an embodiment of the present invention have been described sofar, the above described embodiment is merely an example for carryingout the present invention. Therefore, the present invention will not belimited to the above described embodiment, and can be carried out byappropriately varying the above described embodiment within the rangenot departing from the spirit of the present invention.

1. A method for producing a plug for hot tube-making, comprising a stepof preparing a plug whose surface includes a sprayed coating formedthereon, the sprayed coating containing iron and an iron oxide; and aheat treatment step during which the plug is kept at 400° C. to 550° C.for 5 to 60 minutes.
 2. The method for producing a plug according toclaim 1, wherein the step of preparing the plug further comprises: astep of preparing a plug body; and a step of forming the sprayed coatingby performing arc spraying with an iron wire rod onto a surface of theplug body.